Ok you guys are gonna think I'm crazy. I seriously geeked out on this one. I hope it's somewhat correct...

This one is ceiling mounted, sortof to scale (14" LCD, 10mm lamp arc, 220mm rear fresnel, 330mm front fresnel).

I drew it pretty wide. You might have to scroll to see the screen on the right. ---->

LONG STORY SHORT, it looks like Mikau's real-world result earlier in this thread...
"Perfect! Tilted screen to correct trapezoid, adjusted lens slightly to keep in focus"
... can be successfully modeled using RAYTRACE!

Details:

The animation is six frames, 10s each:
1. initial setup. this should show a "normal" projector. I traced three LCD pixels.
2. tilt the projector 10deg. (actually, I tilted the screen / room).
3. tilt the fresnel 10deg. to correct (definitely improves the image)
4. tilt the fresnel a little more, to 10.9deg (fully fixes trapezoid, BUT not focus)
5. tilt the fresnel way out to 21.3deg. (fully fixes focus, BUT not trapezoid)
6. tilt the fresnel back to 10.7deg. and tilt lens 3 of the triplet (PERFECT IMAGE).
7. I was gonna try tilting the whole triplet, but raytrace crashed on me. maybe tomorrow.

The triplet has a rear positive lens which I focused on the LCD. For the middle (negative) and front (positive) lenses, I just guessed. I played with their focal lengths until I got a reasonable focus, approx. 1m in front of the projector.

I pretended the LCD was black except for three pixels: one red, one green, one blue. I only showed two rays per pixel: a center-ray and a side-ray. Pretty eh? Now let's hope it's correct!...

The lamp rays are shown, cut off at the LCD. I used three lamp raygroups, to represent a 10mm bulb arc. Yes this means non-parallel light hitting the LCD, well I used RAYTRACE's "protractor" tool to figure out, exactly how much non-parallel? 1.302deg, apparently. That's the angle between the "perfect" ray (coming from the middle of the bulb arc) and one of the side rays (coming from the side edge of the bulb arc). Actually that number doesn't really matter for this keystoning discussion, but, it was fun to calculate :-). NOW I see why a perfect point-source bulb wouldn't even need a projection lens... the lens is there to intercept that imperfect 2.604 degree wide cone of light, and squeeze it down onto a single spot on the screen! (Or something like that, anyways :-)

That's actually really, really helpful in describing how the triplet works.

Question: Does one of the fresnels(half fresnels, whatever) used backwards = a diverging thin lense?

Good question. My guess is, if it's converging one way it's also converging the other way

Here's something else from the BenQ website

"Each projector has an individual, optical axis that sets how high an image can be projected above the lens axis. A relationship of 19:1 means that the image will be divided up into 20 parts, 19 of which will be projected above and 1 below the lens central axis.
"With a relationship of 10:1 we talk about 'Con axis', i.e. the projection's lower edge is at the same height as the lens".

http://www.benq.us/serviceandsupport/libra...404,417,425#426

I don't understand it. but it sounds relevant ...

Charlie10,

Nice work. I'm curious though, if you wouldn't mind playing with your raytrace some more to investigate what happens when the triplet lenses are shifted. By this I mean moved up or down, left or right, not tilted and not front to back (keeping all other alignments as if they are straight on projections). I would be curious to see what happens when the entire triplet is thus shifted and what happens when individual lenses within the triplet are shifted. The reason I'm asking is that it seems to me the lens shift mechanism that allows a projector image to hit the screen square, but above the center line, involves moving the triplet and/or individual lenses. I sent Mikau some info from patents involving lens shift and they seem to describe just moving the triplet or one of the triplet lenses up/down/left/right in the same plane as the (untilted) screen.

Thanks,
Chris

So just shifting up/down and not tilting. Sounds good.

The results would be more valid if I had a better more of a clue about the triplet. Pretty sure the middle lens should be negative and the other two positive, but what should the relative focal lengths of the three lenses be?

All three roughly the same?

Charlie10,

You know, I'm not sure what the focal lengths are. If I had to guess, I would say that two of the 3 are probably focused on the lens you call the "rear positive lens" or the other way around and the rear/middle lenses are focused on the front lens. But that's more of a wild guess than anything. Brain would probably know given he originally played with a single throw lens before moving to a triplet that corrects the barrelling of a single lens.

I'll see if I can find info on it elsewhere but if anyone know the answer please do post it!

Thanks,
Chris

Ok I have come up with the following solution:

Joke... you know I did not come up with that. I couldn't resist since the images looked kinda similar. Sorry :-) Anyways that is a cool looking raytrace of a "reverse telephoto" lens I found via google on the following web page:
http://www.cartage.org.lb/en/themes/Scienc.../Lens/Lens2.htm

If you look there is a "Cooke triplet" lens on that page too, which seems closer to what we're doing. So I'll try to get some info from that.

By the way, Google rocks, and so do these forums!

I found a 50mm Cooke triplet design from another program called OSLO. So I scaled this by 7 to get a 350mm triplet, and entered that into RAYTRACE. Well I am not sure about this lens, either that or I got something totally wrong. Because the focal plane came out very distorted-looking, and there's lots of chromatic/color aberration.

Anyways I tried sliding the first and second lens elements up and down, together without tilting, to see what effect this would have. It does appear to have some kind of keystone type effect. However it doesn't seem as compelling. See pic.

(the black lines represent the focal "plane", basically where the screen would go).

edit: tilting the fresnel while sliding just the second lens, seems to be alot more effective. Sorry I don't have the energy to make another GIF of it .

I can't help going back to the initial idea in this thread. Basically sliding/shifting the triplet AND the fresnel up/down by the same amount. It just seems so much simpler than breaking apart the triplet.

For a 330mm triplet, a vertical shift of 80mm / 3" should achieve the equivalent of 15 degrees tilt+keystone-correction.



from http://www.lumenlab.com/forums/index.php?s...indpost&p=57904
please refer to Mikau's upper diagram.

So... do we even need the higher magnification? If the existing triplet has just enough FOV for a 15" LCD, then I guess the extreme corners would almost immediately darken with this type of shifting. However even a slightly smaller LCD should have alot more wiggle room... as long as the shifted rectangle still fits into the 15" circle? Which should be easier the more widescreen the LCD is.

You could even put the whole thing (fresnel+triplet+focus box) on locking sliders, to allow adjustment.

Shouldn't be too difficult to test... anyone up for it?

Nice work Charlie10, that does appear to be "it"! I've seen some 14 and even 12 inch LCDs with the same specs as the usual 15 inch models so hopefully someone has one of these and would be willing to do a little experimenting. Either that or someone with the pro lens and a 15" LCD might try it (although I imagine if they have the pro lens they probably have the larger LCD)....

I tried this out with a single Staples fresnel (page magnifier) and a Surplusshed copy lens triplet. I put a black&white grid pattern on my laptop for an image source.

I can't really verify until I have the proper parts, but even the above crude setup seemed to do a good job of shifting the image up/down/left/right while keeping focus and avoiding keystoning. As long as I moved the lens and the triplet by the same amount. So I'm guessing it could work for smaller LCDs. I can't find many that are <15" though... just laptop LCDs, and nobody seems to use those.

Mikau, what did you use to break apart your triplet? I got the one from surplusshed but it is built pretty solid, I don't think a hacksaw would cut it. The lens retainer ring is threaded, but I don't know how to get enough grip to unscrew it :-/

Hi guys! Sorry I haven't been here for much of this. My computer is in the shop. Its got some new uber spyware. Appearantly a lot of people have it. Not even the people at nortan antivirus have found a remedy yet :angry: blasted spyware!!!

Anyway, nice job charlie, you've done a lot of work. We just need a triplet with double the viewing angle and a matching triplet. Easier said then done..

Thats up to brain.

Anyway, I used a hack saw to take apart my triplet. But mine was plastic. At first I tried to melt them off with a hot knife and almost killed myself with the toxic gas it produced.

Click to view attachmentHas anyone tried using a vesta pocket camera design for their focusing mechanism. This would allow the triplet to be moved around from top to bottom, side to side making. A pic

hello light bulb, fresnel united (they do not incline), tft (if circuiteria inclines next to his) and the objective in its geometrico center. the projector inclines whole, tft arranges parallel to the screen to prove it and to count results.

?????

I'm thinking english isn't his first language. I think that he's saying just tilt your panel.

What would happen if you moved the light down and the triplet up? I have tried this with a framed transparency and an ohp fresnel and it apperars to work.
I will be experimenting some more. I am rebuilding my projector. I am trying to put both fresnel lenses in back of the LCD.

See attached picture...

it happens the same. it is the same system, it requires to move the light bulb and the objective. my single system is necessary to incline tft few degrees. I have not calculated it, about 7 or 10 degrees at the most. I see the advantage him my system no that there is to move the light bulb above and down. the axis of the single light varia. is not necessary to incline a little tft. your system also studies it like alternative, but a little convinces to me but to incline tft. I at the moment cannot test you practice. I recommend to you that you prove systems both. I am almost safe that they work both perfectly, but I have not been able it to prove definitively. my Web so that you see photos it is www.morcillon.tk it inside has carperas to the left with photos. a greeting, luck and place photos of the result.

My Webpage

I've gotten my LL fresnels and triplet, and set up a testbed of sorts. Using an Ikea "Flygel" 50W halogen lamp as a test light source.

Til now I hadn't tried moving the light source. So I tried it. It does seem to work, but maybe at the expense of brightness / uniformity. The beam of light cast after the first fresnel is no longer rectangular in cross section; it's a little squashed on one side, like a twisted piece of paper. The shape of the light hitting the triplet is more quarter-moon than circular. The LCD image was not distorted, but it seemed harder to position the triplet in a way that cast even light. My initial guess is that moving the lamp in the way you describe, could accomplish about 5-10 degrees of shift before it the picture started to degrade... just a guess.

Here's a pic of my most recent setup setup , showing fresnels parallel but offset. A laser level indicates the primary optical axis of the setup, and that laser light gets angled by the second fresnel, hitting the triplet. I got the best results orienting the triplet perpendicular to this light, as shown. The screen is still parallel to the second fresnel.

From what I can tell, the standard LL triplet is able to handle the light path well, even at very significant offset. What I think I need is bigger fresnels.

BO YA! GUESS WHO'S BACK!

I'm pleased to see that this thread stayed alive during my absense, and I think I owe it most to charlie. Thanks, dude!

Well charlie, brain said that the standard triplet is capable of getting about a seventeen inch lcd into focus when the edges will begin to dim. 15 inches is a safe distance from that, but you could probably get away with at least an inch. But thats not much really. What you could do is use a standard 15 inch collinator, and the profield collector, with the pro zoom triplet. The small collector would be better for a 15 inch so no light is wasted, and directly infront you have the pro collector offset as much as possible. The pro collector will cover a 21 inch lcd so you should be able to shift it about 25% which isn't much really, but not too bad when you sit close to the projector and are using a larger projection. (The bigger the projection, the bigger a 25% shift is.)

well, i can focus a 29" flat crt tv with this standar lens as well, it only depends on the trhow. if you project to 2 feet, you will have everithing focused.

I mean that 17" focused is it posible but you need to say what throw was used when tested, this information is very significant.

Rox

Well when I asked brain just said 17 inches, I assume that means at the standard distance for the fresnel.

See that would work if only the throw distance was larger and the projection bigger. But putting it that far away, like you said, will result in a tiny projection at a very short distance. If we had a lens with the same focus area but a longer throw and higher magnification, we'd just need a larger fresnel and use half of it.

the problem with large panels is not the fresnells at all. The main problem is the projection lens. If you get a projection lens with focal and field angle enough then i would say the standar fresnells will do a good job for you. But if you have a lens with a extremelly small field angle, you will have a hard time doing it work.

The fresnell will work on a wide range just moving the lamp placemnt.

I'm talking about for off setting. Lets say you had a 30 inch fresnel and had a triplet that could get a 30 inch lcd into focus and project at an adequete size at an adequete range. Then You could simplay place a 15 inch lcd on the that fresnel on the uper or lower half. This would result in a 50% shift.

i agree. But you would not find such a projection lens.

True.

Hiya Mikau!

Encouraged by this thread and the testing I've done with my halogen lamp (6mm filament) and the standard LL fresnels, I've ordered a pair of large (15"x15") fresnels (from diyprojectioncompany.com). My plan is to place a 9"x12" LCD in one corner of the large fresnel, hopefully achieving about 20 degrees of vertical and 5 degrees of horizontal shift. Will either of my triplets (Fujinon copy lens, Standard LL triplet) be able to focus this shifted image?

So I went ahead and placed the order last week. As of last update it is "Processing". It's not clear to me whether they have the lenses in stock or if there will be a longer delay. The suspense has been killin me!

If the image won't focus, the next step is to break apart the triplet. Would I use a dremel for this?

I'm pretty confident that by hook or crook we can get good results out of this, without needing a pro lens. My latest hope is to incorporate Minoten's panel, and make a nice, small, sharp short-throw coffee-table type setup. I will try to post results for longer-throw setups too.

Well 20% isn't much really, though better then nothing and 20% means a larger shift the bigger the projection.

I think it would be better if you got the pro lens kit. The pro field fresnel will cover up to a 21 inch lcd. Thats six inches more then 15 inches. So you should be able to get at least a 25% shift.

However what I intend to do for my projector is, with the help of an N6, set it to widescreen letterbox display at all times, then I can shift down the fresnel 25%. Then what I believe I can do with the N6 is set full screen to display in letterbox with black bars on the left and right. While resolution is lost, this is still enough for 480p which is about as high as it gets for full screen formats. (Unless you play PC games, which I don't). Like I said, this will give about a 25% shift which will be about 16-18 inches up for a 9-10 foot projection which is what I plan to do. The fullscreen format will be about 1/3 the size this way, but I think I won't mind considering I'll have the projector out of my way.

Later I'll upgrade to a 17 inch panel, with the pro lens kit. The larger then needed fresnel will allow me to off set the panel about 25%. Furthermore I found that most 17 inch panels are actually a 5:4 ratio, (1280x1024) which means its even taller then normal so I can move the fresnel down even farther. So in the end I'll have about a 50% shift which means the projector centered on the top of the screen (which is all you need in my oppinion).

What I wish I could do is make 480p display in 480-640 pixels exactly regardless of the available pixels on my lcd. Then with a 17 inch I could have even higher off setting in fullscreen mode and there would be no distortion from scaling.

Ok, I'm going to continue more talk about what needs to be done ignoring how we could do it.

The reason I don't think we could use a mirror or bend a fresnel is we need to 1, shrink the image on one side and stretch on another, and we also need to change the distance of the screen in differant spots. So in otherwords we need sort of a bent trapazoid. (Though not quite a trapazoid, more like a trapazoid with curved sides)

Though while analysis tells me that bending the fresnel won't work, I can't help but think it just might work. Someone has to try it.

Meanwhile I'm trying to think up cheap means of developing specialized equipment for keystone correction.

The first is a fresnel that is nothing but a bunch of the same triangles. If the projected beam were a square, this would work by placing this fresnel in front of the triplet. Like the picture below shows.

However tilting a triangle is not so easy. Its not exactly a triangle, more like a trapazoid. I need to test it on cad to see if passing through this type of fresnel will not produce a curve.

But even if it did work, redirecting the projector beam mid way through might really mess it up. Might not though.

The other idea is no one here has tried any keystoning experiments with glass wedges. Someone from another site built an annamorphic lens using two prisms, out of pieces of glass, one filled with water, one with oil.

I think if you can build an annamorphic lens you might be able to make a keystone correction device.

This is a simular thing that I was thinking about. I haven’t had time to test the idea yet. Seems to look like it would work.

DJ

Yes I hope so. But optics is quite a delicate buisness.

I made some 15deg prism wedges with lexan and water. A VERY quick atempt a keystone corection left me with some barreling. It wasn't a good test, since it was very quick. I probably was doing something wrong. You are supposed to be able to use 2 wedges to convert an eliptical light source into a circular one.

I need to get back to the experiment, but I did prove its fairly easy to make a test lens system with only some plastic, hot-melt glue gun, and water.

I've been toying with the idea of a varing focus lens. It would be a square with infinite focal length at the top (flat plate) and evenly progress to the desired focal length on the bottom. It would only be in one direction, basicly a cylindrical lens. My method of construction would be either two sheets of lexan with water filling them. One is flat, the other is curved. This would only be a test lens. A rigid lens would be made with a sheet of acrylic curve to shape, and filled with water clear acrylic casting liquid. Neither lens is perfect, but cheap, and easy to make to test out ideas.

Too many ideas... so little time. :-)

Do you have a link to where you found the person making the annamorphic lenses?

Ask joe chevvy, he mentioned it on the inteligent video projector design forum a few weeks backs.

While I think it could possibly be correctable if you have acrilic wedges that you can bend, but bending it both ways would probably be impossible.

Unless perhaps you bent one side one way, and the other another. See picture below.

But you should really try using two normal wedges and goofing around with them.

I really think that the only way we can achieve good keystone corrrection, optically, is if professional equipment is professionally developed specifically for keystone correction/lens shitfing.

I think where we need to go in the future is hope companies develop cheaper smaller lcds with higher specs. So we can offset the lcd on the fresnel. At this point we can still use a 15 or 17 inch with a prolens kit to do this. Also we can use letterbox only if we wish to improve things.

I think that we can do this with current fresnels from diylabs. the 15.5"x15.5" lens should give some shift on a 15" screen.

I also think that keystone is less of an issue with a longer throw. If we could think how to achieve a cheap way for a longer throw that would be a positive effort.

http://www.diyprojectorcompany.com/

W/ longer throw we NEED large + expensive lenses, unfortunately, if we're going to use the same lamp.

If we switched to a smaller arc lamp (example OSRAM's 575W HSD lamps, w/ a 6000hr lifetime, 7mm arc, though I know not the cost), we could use a triplet or achro doublet and not have to worry as much about field angle or focus.

If you ignore the overall arrangement of the pixels for a second, the triplet's job, for each pixel, is to focus the 1" arc into a point in the general vicinity of the screen. With a smaller arc, this is much easier.

With long throws, divergence due to the arc size gets more and more powerful.

Is it true that using a 330mm rear fresnel, instead of a 220mm, will make the bulb arc image smaller?

If so then would such a setup improve focus (I guess at the expense of brightness / light gathered) ?

Apologies if this question has already been answered.

yes, the rate (fiel fresnell/rear fresnell) would give you the magnification of th arc lengh on the projection lens. If you have 330/330 then the arc would be the same size on the triplet (Care this is true only if you are not forcing the field focusing point placing the lamp closer to the rear fresnell, this would have more rate)

but there is other problem using longer rear fresnells, that the light taken from the bulb would be partially wasted because the solid angle will be less on a 330 focal than a 20 focal (the light cone is smaller on 330 than 220, so worst of light)

It would be so great to not waste all that light from the bulb. A pre-condenser lens would help... but would the bulb arc image then become larger again?

thats it. We have then the same problem but on a larger box

Just been thinking.

A normal convex lens will work as a projection lens but produces barreling. Triplets are a special set up of lens designed to correct this aberation. If a round focus plain can be converted to a flat one, why can't a normal focus plane be converted to a tilted focus plain? If glasses can be used to correct imperfections in our eyes, why can't we make a lens to optically correct for keystoning?

I've learned from projector building that optics are an extremly sensitive field and nothing works unless it is crafted with predesigned precalculated precision. Manually readjusting lenses in an attempt to modify its function will result in distortion, in 99.9999% of the cases. While making a keystoning triplet would be easier then offset projection I think, as it wouldn't require a lens with a very large area of focus, it may or may not be possible to design a keystoning triplet that works for every projection size. Then again it very well might, I don't see why the focus plain can't be converted to a tilted one, but what do I know about optics?

At any rate I think if a triplet was designed to only produce keystone correction to a fixed degree (like 40 degrees ) it would work for any projection size, but again thats just speculation.

But if it is indeed possible I don't see why anyone hasn't developed one for projectors of any form. Who does prefer their projector blocking their view? So maybe it is an impossibility. Or maybe no ones really bothered with it. Thanks to things like digital keystone correction.

I hate this. All I can do is speculate. :angry:

All of the recent commercial projectors I use at work have what you describe. The projector sits on a table, and projects perpendicular to the screen. The bottom of the projected image is around the level of the projector, and the rest goes up (or down if inverted for ceiling mounted) Everything is in focus. Must be custom optics.

I think this is what you were getting at... now if I/we can only come up with the solution.

Perpendicular to the screen would mean lens shift, not keystone correction.

Thats another possibility, once again I think it would be cheaper if it was shifted only to a specific distance, this would probably require a front lens being abnormally large.

Its definitly possible, but someone has to convince an optics company to develop one first, and even then it might be a costly sophisticated triplet.